BELGRADE, DEC. 10 -- Serbian President Slobodan Milosevic, a hard-line former Communist who built his power base on impassioned appeals to Serbian nationalism, won a resounding victory in Sunday's election in Yugoslavia's largest republic.

Election officials tonight projected that Milosevic would win about 60 percent of the vote, defeating his nearest rival by a ratio of nearly 4-to-1. Opposition leaders conceded defeat tonight while complaining of vote rigging, voter intimidation and manipulation of the state-owned Serbian media.

Early returns also indicated that the Serbian Socialist Party, which is rigidly controlled by Milosevic, was running well ahead in voting for the republic's 250-seat legislature.

The overwhelming Socialist victory, the scale of which clearly stunned the opposition, sets Serbia apart from four other Yugoslav republics that earlier this year dumped former Communist parties.

In the small republic of Montenegro, which is closely allied to Serbia and also voted on Sunday, the Communist Party also appeared to be winning by a wide margin.

The victory for Milosevic, whom Western governments consider the individual most responsible for inflaming ethnic tensions in this country, seems likely to accelerate what the U.S. Central Intelligence Agency recently predicted would be the breakup of this ethnically fractured Balkan country.

Serbs are the largest ethnic group in Yugoslavia, and they share a collective sense that their interests were neglected by the late Communist dictator Marshal Tito, who ruled here for 35 years until his death in 1980. Milosevic rose to power three years ago by promising Serbs that he would be their champion.

By doing precisely that, the Serbian president infuriated three other major ethnic groups and precipitated demands for a dismantling of the Yugoslav federation.

Slovenia, the richest and most northerly republic, decided last week to hold a referendum on independence. The vote will take place in two weeks, and pollsters predict Slovenians will vote overwhelmingly for independence.

Fearing federal army interference in their affairs, Slovenian authorities last week posted heavily armed soldiers from the republic's militia around major public buildings. That move came after the Yugoslav defense minister threatened to use the federal army, which is dominated by Serbian officers, to disarm police and militia in the republics.

The other Yugoslav flashpoint is Kosovo, a nominally "autonomous" Serbian province, which Milosevic describes as the sacred homeland of Serbs but where nine out of 10 people are ethnic Albanians.

By insisting on complete police and political control in Kosovo, Milosevic's government has imposed what international human rights groups condemn as severe repression and widespread human-rights abuses.

To protest Serbian policy in Kosovo, almost all of the 700,000 Albanians who are registered to vote there refused to go to the polls on Sunday. Unlike his major opponents, Milosevic has refused any dialogue with the leaders of the northern republics or with the Albanians in Kosovo.

Opposition parties tonight admitted that they were surprised and disappointed by the size of their loss. They claimed that the election campaign was marred by an atmosphere of fear, by vote rigging and by the ruling party's control of state radio, television and newspapers in Serbia.

Their complaints were supported, in part, by the report of a team of American election observers. The National Republican Institute for International Affairs, a Republican Party group funded by Congress, said that voting-day procedures were generally fair. But it added that observers had heard repeated complaints about "bias and favoritism in media coverage of the campaign."

The group said that the complaints "raised serious concerns about the fairness of the election campaign. . . . They were widespread enough to warrant careful investigation."

The most impassioned complaint about Sunday's voting came from presidential candidate Vuk Draskovic, a novelist and fervent nationalist who preelection polls had shown might have challenged Milosevic.

"The official results show that the citizens voted for slavery, Bolshevism, the past, darkness and disgrace," said Draskovic.

Election commission figures showed that Draskovic, a theatrical public speaker who wears the beard and long unkept hair of a 19th-century Serbian patriot, won only about 17 percent of the vote. "We don't have any guarantee {now that the opposition has lost} that the Bolshevik propaganda machine will change or that the propaganda won't increase," Draskovic said. "I ask myself as a free man why I should be a part of such a Serbia. I will no longer belong to it."

Draskovic said, however, that he and the other major opposition parties would now join forces to try to win as many legislative seats as possible in the second round of voting, which is due in two weeks.

In districts where no candidate won a majority, the two leading vote-getters will compete in the runoff. Opposition spokesmen said tonight that they would be moving into the second round in at least 24 districts.